zwarefermion

Zwarefermion is a term used in condensed matter physics to describe a class of intermetallic materials in which electrons behave as if they have an effective mass greatly larger than the free electron mass. This heavy mass originates from strong electronic correlations between localized f electrons, typically from rare-earth or actinide ions, and itinerant conduction electrons.

In many zwaard fermion systems, localized moments from f electrons hybridize with conduction bands, forming a Kondo lattice. At low temperatures, Kondo screening and coherent hybridization lead to a collective state with a highly enhanced density of states at the Fermi level. This results in a large effective electron mass and distinctive thermodynamic and transport properties.

Key signatures include a large electronic specific heat coefficient gamma, often in the tens to hundreds of millijoules per mole-K^2, and an enhanced magnetic susceptibility. Experimental probes such as de Haas–van Alphen measurements reveal very large effective masses, while electrical resistivity and optical conductivity reflect nontrivial scattering and coherence effects.

Common materials that exhibit heavy-fermion behavior include compounds based on Ce, Yb, and U, such as CeCu6, CeAl3, CeCoIn5, YbRh2Si2, and UPt3. These systems can display Fermi-liquid behavior with a high effective mass, or, near quantum critical points, non-Fermi-liquid behavior. Some heavy-fermion compounds also become unconventional superconductors at very low temperatures, with pairing mechanisms linked to strong spin fluctuations.

Experimental study of zwaard fermion involves measurements of specific heat, magnetic susceptibility, transport, quantum oscillations, neutron scattering, and photoemission to map the hybridization and many-body state. See also heavy-fermion systems, Kondo effect, quantum criticality, and unconventional superconductivity.